70 BELL, 



In other words he assumed without real investigation that the quan- 

 tity which we now know as v was a constant for all substances. 



Just how this singularly maladroit piece of experimentation leaked 

 out is unknown, although the fact is stated in the most positive manner 

 both by Sir David Brewster, Newton's admiring biographer, and by 

 Sir John Herschel. It is not unlikely that the fact got abroad at 

 Cambridge in Newton's later years, and was passed along to Sir 

 William Herschel. Certainly, as one of Newton's later apologists 

 naively suggests, the fact was not recorded in Newton's "Opticks." 



Be that as it may, it was the kind of thing for which a second-year 

 student in physics would get a wigging which would linger long in his 

 memory, but a blunder with a great name behind it carries far, and in 

 this case it put off for a couple of generations the discovery of the 

 principle of achromatism. 



Following his error, Newton gave up all idea of an achromatic lens, 

 and turned his attention to reflectors, apparently being unacquainted 

 with, or entirely disregarding, what James Gregory had done before 

 him. Newton had taken to the country on account of the plague, and 

 only about 1670 did he apparently begin to revolve in his mind a re- 

 flecting telescope. It was 1672, January 11th, when he presented to 

 the Royal Society a small model of his preferred form of reflector, 

 which is still in the possession of the Society. 



This little model had an aperture of about 1" and focal length of 

 about 6", and magnified some 38 diameters. Newton was able, he 

 says, to detect with it the moons of Jupiter and the horns of Venus, 

 both feats which can be accomplished with the very slightest of 

 optical aid, providing the magnification is anywhere near that which 

 Newton used. But here again Newton gave notable evidence of his 

 unpractical experimenting, clever though he was. 



He had firmly fixed in his mind the entirely erroneous idea that a 

 spherical mirror was quite good enough for the purpose, even when an 

 aperture of F/6 or F/8 was used, believing that the trouble with the 

 long telescopes of the day was almost entirely their chromatic aber- 

 ration. This was partly true, but the spherical aberrations would 

 have been equally bad save for the very narrow aperture employed. 



This error would certainly have brought Newton to grief had he 

 attempted a telescope of any perceptible size and in fact there is some 

 evidence that this actually happened. On January 25th, just two 

 weeks after his model was displayed, the minutes of the Royal Society 

 note that: "There was produced a reflecting telescope 4 feet long of 

 Mr. Newton's invention which though the metaline concave was not 



